1. Field of the Invention
The present invention relates to a power supply device for driving a machine tool and the like, and more particularly to an outage management emergency power supply device for supplying the energy required for outage management during a power failure.
2. Description of the Related Art
For a machine tool and the like, a variety of measures are carried out for dealing with an outage when a power failure occurs. As control items implemented in accordance with outage management, there are retract control, brake control, and drop prevention control.
In a numerically-controlled machine tool in which a tool and a workpiece are synchronized, when a power failure occurs, retract control maintains the synchronization of the workpiece and tool, and removes them to a location where they will not interfere with one another. In accordance with utilizing this control, damage caused by workpiece-tool synchronization drift is prevented. Machine tools, which perform this kind of retract control, include hobbing machines, gear grinding machines, polygon finishing machines and the like.
In machine tools for which the coasting distance of the feed rod during a power failure poses a problem, brake control performs braking and stopping so that a collision does not occur as a result of the inertial run of the feed rod.
For machines equipped with a gravity axis, drop prevention control maintains the present location so that the gravity axis does not drop when a power failure occurs and cause damage to the workpiece and tool.
Driving energy is required to carry out the above-described outage management, but since the driving energy supplied from the power source side during a power failure falls off abruptly, an emergency power supply device is required to supplement the deficiency by supplying energy.
The supply of deficient energy during outage management has in the past been carried out by an uninterruptible power supply (UPS) connected to a converter input portion.
A well-known UPS is a system called an M-G system, which combines a flywheel and a generator. With this M-G system, the flywheel is made to rotate under normal conditions, and when a power failure occurs, the flywheel drives the generator, and the obtained output is used as backup power.
The problems with utilizing a UPS as the supply source for energy needed during outage management following a power failure are that the constitution of the system is complex, and it requires maintenance, and the costs associated with the system itself and maintenance mount up. The fact that it requires a large installation area is also a problem.
When a relatively little energy is needed to deal with an outage, the deficient energy can be supplied by series connecting an auxiliary capacitor to a DC link portion, which is a direct current voltage supply line connected to the output of a converter.
FIG. 1 is an example of the constitution of an outage management emergency power supply device, which makes use of an auxiliary capacitor. In the constitution example of FIG. 1, a converter device 2 comprises a converting circuit 21 and a smoothing capacitor 20. The converting circuit 21 comprises a rectifier portion which converts an alternating-current power supply to direct current. The smoothing capacitor 20 is connected to the converting circuit 21 through a parallel circuit composed of a charging current-limiting resistor 22, a diode 23, and a thyristor 24. The connection terminal of the smoothing capacitor 20 constitutes a DC link portion 4.
An inverter device 3 comprises a converting circuit 31 and a smoothing circuit 30, receives direct-current current from the DC link portion 4, convert the DC current to alternating current, and supplies driving current to a motor 5.
An auxiliary capacitor 10 is connected to the DC link portion 4 of this converter device 2, and energy needed for outage management can be supplied by storing up energy in the auxiliary capacitor 10 during normal operation, and discharging energy therefrom when a power failure occurs.
When a lot of energy is required for outage management following a power failure, this can be dealt with by increasing the capacity of the auxiliary capacitor connected to the DC link portion 4, but the capacity of an auxiliary capacitor is limited. The reason for this is explained below using the circuit diagrams of FIG. 2A and FIG. 2B.
The first reason for this has to do with a problem that occurs at start-up of a converter device. When the converter device starts, an emergency charging current flows to the smoothing capacitors 20 and 30 as indicated by the arrows in FIG. 2A, and current is supplied to the auxiliary capacitor 10 from the DC link portion 4. When this happens, if the auxiliary capacitor 10 has a large capacity, the emergency charging current flowing to the smoothing capacitors 20 and 30 will be reduced, lengthening the time required for emergency charging. Consequently, to complete emergency charging within the prescribed time, the capacity of the auxiliary capacitor must be limited.
Further, because the current flowing to the charging current-limiting resistor 22 increases in line with the increase in capacity of the auxiliary capacitor 10, a charging current-limiting resistor which can stand a large current must be utilized, and the capacity of the auxiliary capacitor is limited by the size of the charging current-limiting resistor 22 used, and such set conditions as price.
Another reason has to do with a problem that occurs during an outage. In FIG. 2B, when a brief power failure occurs which is of such short duration that machine tool operation is not interrupted, because the energy supply from the power source line is cut off momentarily, DC link voltage decreases. When the duration of a brief power failure is long, the capacitors are charged during recovery in accordance with the voltage difference between the decreased DC link voltage and the recovered power source voltage. Because the charge performed at this time does not pass through the charging current-limiting resistor 22, a rush current flows into the capacitors (Refer to arrows in FIG. 2B). Since this rush current is proportional to the capacity of a capacitor, when the capacity of a capacitor is large, the current flow is excessive. This excessive current runs the risk of damaging converter input power components.
Consequently, the capacity of the capacitor connected to the DC link is limited in accordance with the tolerance of the converter input power components.